Hi there. I have only skim-read the thread, but just wanted to add my support/advice.
I work with young people (13-19 yr olds), including young parents and young women who have decided to terminate when faced with an unplanned pregnancy.
I can only tell you what I would do (as a professional. I would do this as a parent, too).
Firstly, I would keep the lines of communication as open as possible - and you can only do that if you present a non-judgemental, supportive face to your DSD. If she senses you are not on side, she won't be open with you, and that could cause further problems down the line.
Secondly, I would sit her down (you and your DH, as calmly as possible) and explain to her that, if she feels responsible enough to have this baby, she owes it to herself and her unborn child to at least consider all the options that might be open to her.
Nobody can force her into anything, and she shouldn't be pressurised (that would be very wrong) - but she should at least familairise herself with the options. A good clinic, like Brook Advisory or similar, can do this, and can also offer her counselling (even just a one-off) if she wants to talk to someone in confidence outside of the family about this situation.
From there on in, you have no option really but to support her. She does need to be made aware of what the implications of her decision to have this baby will be - interupted education, limited social life for a few years, the end of her childhood, to put it bluntly - but not in a scaremongering way. If she is going to be bring a baby into this world at her age, she is going to have it hard enough.
The biggest influence of how well young parents get on is the level of support they have from family. This will change her life, but with your support, her life is by no means over.
Good luck